1. Field of the Invention
The present invention relates to a cellular communication system and, in particular, to methods and apparatuses for transmitting and receiving downlink common channels such as synchronization channel and broadcast channel in a cellular communication system supporting bandwidth scalability.
2. Description of the Related Art
Recently, Orthogonal Frequency Division Multiplexing (OFDM) is becoming very popular for broadcast and communication systems. OFDM is advantageous to reduce Intersymbol interference and fading caused by multipath propagation and improve spectral efficiency with a large number of closely spaced orthogonal subcarriers. With these advantageous features, OFDM is regarded as a promising solution for high speed data transmission and broadband communication system and superior compared to Direct Sequence Code Division Multiple Access (DS-CDMA) technology.
FIG. 1 is a diagram illustrating an OFDM-based downlink frame structure in Evolved Universal Terrestrial Radio Access (EUTRA) specified in the 3rd Generation Partnership Project (3GPP) standards. Referring to FIG. 1, the 20 MHz system bandwidth 101 is divided into 100 Resource Blocks (RBs) 105. An RB consists of 12 consecutive subcarriers 103 by 14 OFDM symbol periods. Each subcarrier 103 for one OFDM symbol duration carries a modulation symbol of downlink channel. Each box within the resource grid representing a single carrier for one symbol period is referred to as a Resource Element (RE) 106. In FIG. 1, the RB is composed of total 168 REs (14 OFDM symbols×12 subcarriers). A single downlink data channel can be assigned one or more RBs for one OFDM symbol duration 104 according to the data rate.
In the cellular communication system, bandwidth scalability is one of the key performance attributes for providing high speed wireless data service. For instance, the Long Term Evolution (LTE) system supports various bandwidths of 20 MHz, 15 MHz, 10 MHz, 5 MHz, 3 MHz, and 1.4 MHz as shown in FIG. 2. Accordingly, the LTE service provider can select one of the available bandwidths, and a mobile terminal also can be configured to support various capacities of 1.4 MHz to 20 MHz bandwidth. In order to fulfill IMT-Advanced requirements, LTE-Advanced (LTE-A) supports carrier aggregation to allocate up to 10 MHz.
In the system supporting the bandwidth scalability, the mobile terminal is required to be able to carry out the initial cell search without information on the system bandwidth. The mobile terminal can acquire synchronization to the base station and cell ID for demodulation of data and control information through cell search procedure. The system bandwidth information can be acquired from the Synchronization Channel (SCH) in the cell search procedure or by demodulating a Broadcast Channel (BCH) after the cell search procedure. The BCH is a channel used for transmitting the system information of the cell which the mobile terminal accesses and is demodulated first right after the cell search procedure. The mobile terminal can acquire the system information such as the system bandwidths, System Frame Number (SFN), and physical channel configuration of the cell by receiving a shared control channel.
FIG. 2 is a diagram illustrating an exemplary frequency resource mapping of SCH and BCH according to a system bandwidth in a conventional system supporting bandwidth scalability. The mobile terminal performs cell search on the SCH and, once the cell search has completed successfully, acquires the system information on the cell through the BCH. In FIG. 2, the horizontal axis 200 denotes frequency in MHz, and the SCH 204 and BCH 206 having 1.08 MHz bandwidth are transmitted in the middle of the system bandwidth regardless of the bandwidth scale. Accordingly, the mobile terminal can find the RF carrier 202 regardless of the bandwidth scale of the system and acquire an initial synchronization to the system by performing the cell search on the SCH 204 defined by 1.08 MHz bandwidth centering on the RF carrier 202. After finding the cell, the mobile terminal demodulates the BCH 206 transmitted within the same 1.08 MHz bandwidth centering on the RF carrier 202 to acquire the system information.
FIG. 3 is a diagram illustrating a frame format of a 10 ms radio frame of an LTE system in which the SCH and BCH are transmitted. The SCH is transmitted in the forms of Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) on every 0th subframe (subframe #0) and every 5th subframe (subframe #5). Each of the PSS and SSS has a length equal to an OFDM symbol duration and transmitted through 1.08 MHz bandwidth in the middle of the system bandwidth 303 as shown in FIG. 2. The BCH 302 is transmitted for four OFDM symbol durations within the subframe #0.
The LTE-A system supports a bandwidth wider than the LTE system for supporting high speed data transmission and should be implemented to provide backward compatibility for the LTE terminals to access the LTE-A system. For this purpose, it is required to divide the downlink band of the LTE-A system into subbands for the LTE terminals and transmit the SCH and BCH through all the subbands. In this case, however, the SCH and BCH are transmitted redundantly from the viewpoint of the LTE terminal having the capability supporting the entire system bandwidth of the LTE-A system.